Hydrology, Cryosphere & Earth Surface News

The Legacy of Nitrogen Pollution

Researchers track decades of nitrogen inputs and uptake across the United States, highlighting the need for policy to address the legacy effects of this essential nutrient and pollutant.


Kim Van Meter jokes that she can trace her career back to a family road trip across the Midwest she took as a child, when an aunt promised a restless Van Meter a penny for each cow she counted along the way. Van Meter dutifully tallied up thousands of cows in pastures along the roadside, collecting all of her aunt’s poker money at the end of the trip.

More recently, as an assistant professor of ecohydrology at the University of Illinois at Chicago, Van Meter found herself counting cattle again, this time to help create an unprecedented database of nitrogen inputs and outputs across the United States dating back to the 1930s, which could help researchers and policymakers understand nitrogen pollution and how to address it.

Nitrogen is not like most other environmental pollutants, in that we need it to feed the world. “Half of the food that we eat wouldn’t be around if it weren’t for synthetic fertilizer,” said David Kanter, an assistant professor of environmental studies at New York University who was not involved in the study. “It’s essential, but at the same time our inefficient use of it makes it one of the major environmental problems of our time.”

Nitrogen-rich runoff from farms and urban areas has poisoned groundwater and led to oxygen-deprived dead zones and toxic algae in water bodies. Nitrogen also forms a potent greenhouse gas that adds to global warming. And although agriculture is the dominant source of nitrogen pollution, it’s not the only one. Indeed, nitrogen has such a large number of sources and environmental impacts that it’s been a major challenge for both researchers and policymakers to get a full picture of the issue.

In the new study, published recently in Global Biogeochemical Cycles, Van Meter and her coauthors tallied up 8 decades of nitrogen inputs (from fertilizer, biological processes, manure, and human waste) and uptake by crops to determine just how much excess nitrogen may be present in counties across the United States.

“The fact that they manage to collect such a multiplicity of sources at such a fine-grain scale, and particularly over such a long time period, is impressive,” said Kanter.

Much of the data were drawn from the U.S. Department of Agriculture’s agricultural census and transferred into spreadsheets to get the numbers into a form the team could work with.

“We had an army of undergrads, going line by line transcribing all the numbers, every small detail for orange trees, cows, chickens,” said Danyka Byrnes, a doctoral student at the University of Waterloo and first author of the study. It was daunting and, at times, incredibly tedious work—but it wasn’t mindless. Over the years, the census categories evolved as the purpose of the census, the leadership, and the country itself changed. “It wasn’t just pulling the numbers, but thinking about how the census changed and reported data in different ways over different decades and years,” said Nandita Basu, an associate professor of water sustainability and ecohydrology at Waterloo and a study author.

The Case of the Missing Cows

The team focused on county-level data because that’s the level at which most agricultural data were reported. But not every category of information, such as the number of cattle or the atmospheric deposition of nitrogen, was available at that fine-grain level for the entire study period, from 1930 through 2012. Often, data resolution increases over time, as the methods and technologies researchers use to measure variables become more precise. But the team encountered the opposite trend for livestock data.

The authors had watched livestock numbers rise steadily after the 1930s and 1940s in some counties across the United States. Then, very suddenly, the cattle disappeared from the county-level data.

The researchers were baffled, until they learned that the agricultural census sometimes suppresses data to protect the privacy of individual farmers. As livestock farming shifted from several small-scale operations to just a small number of farmers raising large numbers of cattle or chickens, the census stopped reporting those data at the county level.

“We had to find ways to not let that suppressed data escape our detection, so we would look at state-scale data or even national-scale data to help us find the missing cows or the missing chickens,” said Van Meter. “It’s like putting puzzle pieces together.”

When the group looked at various parts of the country with widely variable characteristics, they found that nitrogen had a homogenizing impact. The team compared three counties in Washington, Iowa, and Southern California—regions with widely variable climates, crops, and natural landscapes—and found that the nitrogen surplus numbers were roughly the same. “It makes clear how these artificial inputs of nitrogen, these commercial nitrogen fertilizers, they can completely homogenize landscapes that in all other ways would look completely different from each other,” Van Meter said. “That is the power of the nitrogen.”

But it’s not yet clear how nitrogen pollution affects those various regions, according to Kanter. Do nitrogen surpluses translate to more nitrogen pollution in the atmosphere, reducing air quality, or does nitrogen wind up in rivers and lakes, affecting water quality? Those are questions that this data set might help answer going forward. “This is a really useful long-term data set, but what needs to be done now is to connect it to the potential impacts, because that’s ultimately what policymakers and politicians care about,” Kanter said.

A Legacy of Pollution

The data set also helps to quantify the legacy impacts of nitrogen, which can stick around for decades in the environment, frustrating policymakers who have spent significant time and resources trying to reduce nitrogen runoff from farms, with little to no improvement in water and air quality.

“There’s this frustration that keeps growing because we’ll reduce the amount of fertilizer we use, we’ll put in cover crops, and things don’t improve, or they don’t improve as quickly as we think they’re going to,” Van Meter said. “This lack of immediate response is getting more people to realize that there are these legacy effects. The water quality we’re seeing today is not caused by what we’re doing this year or even what we’ve done the last 5 years. There’s decades of history behind what we’re seeing today.”

Understanding the legacy impacts of nitrogen will change the way we work to address nitrogen pollution, broadening our focus from the farm to include the areas where the nitrogen ends up.

The biggest benefits of addressing nitrogen pollution are local improvements to water and air quality, but doing so also has important climate implications. Nitrous oxide is a greenhouse gas some 300 times more potent than carbon dioxide. “What’s difficult about convincing a national or global population when dealing with climate change is that the benefits of reducing greenhouse gas emissions are shared globally, where the costs are felt locally, so you don’t necessarily see much bang for your buck at home,” Kanter said. The fact that the local benefits of addressing nitrogen pollution outweigh the global benefits makes it “one of the most politically feasible climate mitigation strategies today.”

—Kate Wheeling (@katewheeling), Science Writer

Citation: Wheeling, K. (2020), The legacy of nitrogen pollution, Eos, 101, https://doi.org/10.1029/2020EO150644. Published on 21 October 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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